Abstract
Parasympathetic innervation of meninges and ability of carbachol, acetylcholine (ACh) receptor (AChR) agonist, to induce headaches suggests contribution of cholinergic mechanisms to primary headaches. However, neurochemical mechanisms of cholinergic regulation of peripheral nociception in meninges, origin place for headache, are almost unknown. Using electrophysiology, calcium imaging, immunohistochemistry, and staining of meningeal mast cells, we studied effects of cholinergic agents on peripheral nociception in rat hemiskulls and isolated trigeminal neurons. Both ACh and carbachol significantly increased nociceptive firing in peripheral terminals of meningeal trigeminal nerves recorded by local suction electrode. Strong nociceptive firing was also induced by nicotine, implying essential role of nicotinic AChRs in control of excitability of trigeminal nerve endings. Nociceptive firing induced by carbachol was reduced by muscarinic antagonist atropine, whereas the action of nicotine was prevented by the nicotinic blocker d-tubocurarine but was insensitive to the TRPA1 antagonist HC-300033. Carbachol but not nicotine induced massive degranulation of meningeal mast cells known to release multiple pro-nociceptive mediators. Enzymes terminating ACh action, acetylcholinesterase (AChE) and butyrylcholinesterase, were revealed in perivascular meningeal nerves. The inhibitor of AChE neostigmine did not change the firing per se but induced nociceptive activity, sensitive to d-tubocurarine, after pretreatment of meninges with the migraine mediator CGRP. This observation suggested the pro-nociceptive action of endogenous ACh in meninges. Both nicotine and carbachol induced intracellular Ca2+ transients in trigeminal neurons partially overlapping with expression of capsaicin-sensitive TRPV1 receptors. Trigeminal nerve terminals in meninges, as well as dural mast cells and trigeminal ganglion neurons express a repertoire of pro-nociceptive nicotinic and muscarinic AChRs, which could be activated by the ACh released from parasympathetic nerves. These receptors represent a potential target for novel therapeutic interventions in trigeminal pain and probably in migraine.
Highlights
In migraine pathophysiology, much attention was traditionally paid to the role of trigeminal innervation of meninges, likely representing a triggering zone for migraine pain [1,2,3]
It has been shown that parasympathetic fibers originating from the parasympathetic sphenopalatine ganglion (SPG) can innervate and interact, via released acetylcholine (ACh), with somatic trigeminal nerves located around essential meningeal vessels [5]
This ACh analog is an effective vasodilator of cranial vessels [11]. It has been shown [12] that injection of botulinum neurotoxin/A (Botox), which inhibits the exocytosis of ACh, can reduce plasma protein extravasation, a phenomenon often associated with migraine
Summary
Much attention was traditionally paid to the role of trigeminal innervation of meninges, likely representing a triggering zone for migraine pain [1,2,3]. It is known that carbachol, a typical agonist of ACh receptors (AChRs), can induce headaches, but not migraine-like attacks, in patients with migraine without aura [10]. This ACh analog is an effective vasodilator of cranial vessels [11]. Whereas nicotinic ACh receptors (nAChRs) are likely involved in pathophysiology of neuropathic pain [14], the anti-nociceptive rather than pro-nociceptive action of nicotine has been reported in in vivo pain models [15,16,17]. Parasympathetic innervation of meninges and ability of carbachol, acetylcholine (ACh) receptor (AChR) agonist, to induce headaches suggests contribution of cholinergic mechanisms to primary headaches. Neurochemical mechanisms of cholinergic regulation of peripheral nociception in meninges, origin place for headache, are almost unknown
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